Method and apparatus for generating coverage in a cellular network

ABSTRACT

A sub-node, such as a repeater station or relay station, equipped with a multi-beam sub-node link antenna. The beams of the sub-node link antenna allow communication between the sub-node and at least two nodes (access points/base stations)  11   A-B   ; 11   A-E . The sub-node is further provided with at least one transmitting and receiving coverage antenna to generate coverage in at least one geographical secondary coverage area a; a-c, and a control mechanism that selects one of the nodes to extend the geographical primary coverage area A-B; A-E of the node  11   A-B   ; 11   A-E  to include the geographical secondary coverage area a; a-c of the sub-node. The invention is also related to a cellular communication network, and a method for generating coverage in a cellular communication network.

TECHNICAL FIELD

The present invention relates to a method and apparatus for improvingcoverage in a cellular network.

BACKGROUND

Wide-area coverage is achieved in a number of ways in today's systems.Examples of existing access point coverage solutions include high-gainantennas, higher-order receive diversity, transmitter coherent combining(TCC), and high-altitude antennas (up to hundreds of meters aboveground).

All these solutions are based on adapting the access point configurationto enhance the coverage. On downlink, increased EIRP (EffectiveIsotropic Radiated Power), as provided by high-gain antennas and TCC,gives increased signal strength at the terminal (user equipment). Onuplink, high-gain antennas and higher-order receive diversity provide anincrease in effective access point receiver sensitivity by extractingadditional signal energy using effectively larger antenna apertures.Finally, high-altitude antenna installations provide enhanced coverage(on downlink and uplink) by reducing the path loss.

The solutions outlined above provide increased coverage, but often witha marginal rate of return that approaches zero (or becomes small enoughnot to warrant implementation of the solution) when the desiredimprovement in coverage is significant, say tens of dBs or more in termsof signal strength. The drawbacks of some existing solutions, in termsof their providing major coverage improvements, are presented below.

High-gain antennas derive their high gain from a decrease in half-powerbeamwidth (which is directly related to the antenna size). However, thesmallest useful beamwidth is limited by the angular spread of thepropagation environment, which means that the effective installedantenna gain becomes significantly lower than that of the antenna infree space when the (free-space) beam is too narrow. The smallest usefulbeamwidth, in the elevation plane, is also limited by the expected swayprofile of the tower or mast: the beam must be wide enough to maintainproper illumination of the desired coverage area within the interval ofrealized pointing directions that result from tower or mast movement.High-gain antennas can provide coverage improvement on both uplink anddownlink.

Transmitter coherent combining (TCC) is based on using multiple (power)amplifiers in parallel and combining their output signals to generate aneffective output power equal to the sum of the output powers of theindividual amplifiers. One drawback of additional amplifiers isincreased energy consumption related to running and cooling theamplifiers, which gives increased OPEX. TCC is a downlink-only methodfor coverage improvement.

Higher-order receive diversity works by extracting signal energy usingmultiple sensors (antennas) at different locations, in differentdirections, and/or with multiple polarizations. It requires anadditional uplink radio chain (including antenna, tower-mountedamplifier, feeder, and radio) for each additional sensor, which givesincreased capital expenditures. It also requires larger thanconventional cabinets to accommodate the extra receiver equipment.Higher-order receive diversity is an uplink-only method.

The hitherto described methods for generating increased coverage(wide-area coverage) share the common drawback that a 3 dB increase ofcoverage requires a doubling of the “equipment”: the area of high-gainantennas must double for every additional 3 dB of gain, TCC requirestwice as many amplifiers for a 3 dB gain, and twice as many receiverradio chains are needed to get a higher-order receive diversity gain of3 dB (ignoring gain due to fading statistics, which approaches zero whenthe number of receiver chains is large). Obviously, there is a limit forany practical application at which the cost, i.e. capital expenditures(CAPEX) and/or operational expenditures (OPEX), and sheer volume andweight of the equipment make these types of coverage solutionsunsuitable.

Yet another coverage method is high-altitude antennas which improve thepath loss by providing line-of-sight propagation to a larger part of thecoverage area, be it directly to terminals or to reflection/diffractionpoints in the environment. Because of the large distance to ground, thesignal correlation over the antenna aperture may also be improvedresulting in higher effective gain (approaching the free-space gain).However, high-altitude antennas require high masts or towers and mayrequire long feeder cables. The former can make the total access pointvery expensive (CAPEX), whereas the latter can be both costly andinefficient due to transmission losses in the feeders (CAPEX and OPEX).High-altitude antennas can provide coverage improvement on both uplinkand downlink.

In conclusion, present access point-based coverage solutions can provideimproved coverage, but become increasingly inefficient as the coveragerequirements are raised.

Traditional repeaters are also used to create coverage. However, atraditional repeater uses one single sub-node link antenna with a singlemain beam for communication with one specific access point. This can bea very poor solution in many systems. For example, in CDMA systems, aproperty called cell breathing is common. Cell breathing refers to the(slow) dynamic expansion and contraction of the footprint of a CDMAcell, which may depend on the number of users connected at any givenmoment or, in general, the traffic load in the cell and which can beused to balance the load between neighboring cells. Pro-active cellbreathing (and cell optimization in general) can be achieved by forexample tuning of pilot power and antenna tilt. Since a traditionalrepeater provides coverage for a fixed area, it defeats the purpose ofpro-active cell breathing by always providing coverage over a particulararea for the same access point. In addition, the quality of thecommunications link between access point and repeater is affected by thecell breathing, when cell breathing is performed using power control ofcell-defining pilot signals or antenna tilt. In this case, theperformance in the area covered by the repeater may show unacceptablylarge fluctuations.

An example of a prior art bidirectional repeater for wirelesscommunication systems is disclosed in EP 1 445 876, assigned toCalifornia Amplifier Inc. The disclosed repeater is provided with a linkantenna to establish communication with a dedicated base station (accesspoint) and a bidirectional coverage antenna to generate coverage in ageographical area poorly covered, or not covered at all, by the basestation coverage antenna. This type of repeater may be used in acommunication system as shown in FIG. 1. One base station 2 _(A) isprovided with an antenna system 3 that generates coverage to ageographical primary coverage area “A”. The same antenna system 3(including transmitting and receiving antennas) communicates via signals4 with a repeater 5 _(a). The repeater 5 a receives and transmitssignals to the base station 2 _(A) using a link antenna 6, and generatescoverage to a geographical secondary coverage area “a” using a coverageantenna 7. A terminal 8 communicates via signals 9 with the repeater 5_(a) using the coverage antenna 7, and a communication link between theterminal 8 and the base station 2 is established through the secondarycoverage area a provided by the repeater.

In U.S. Pat. No. 4,727,590, by Minori Kawano et al., a repeater isdisclosed having a link antenna to establish communication with one ormore dedicated base stations (access points) and a receiving antenna toreceive signals in up-link from a terminal close to the repeater anddirect the signals to the base station closest to the terminal. Theterminal receives signal in down-link directly from the base stationcoverage antenna. This type of communication network is shown in FIG. 2comprising three base stations 2 _(A), 2 _(B), 2 _(C). Each base stationis provided with an antenna system 3 _(A), 3 _(B), 3 _(C) that generatescoverage to geographical primary coverage areas A, B and C,respectively. These primary coverage areas are normally overlapping, buta straight line is drawn for illustrating purposes. A repeater 5 _(abc)is arranged at a position with equal distance to the three basestations, and all three base stations communicate with the repeaterindependently of each other. The repeater 5 _(abc) is provided withreceive coverage antennas to receive signals from terminals 8 close tothe repeater 5 _(abc), and transmit link antennas to communicate withthe base station. Only secondary reception coverage areas a, b and c arethus generated by the repeater. In up-link, a terminal 8 arranged withinsecondary reception coverage area b transmits a signal to the repeater 5_(abc), and the repeater then forwards an amplified signal to the basestation 2 _(B). In down-link, the base station directly transmits asignal to the terminal 8.

A problem with the existing repeater stations, or relay stations, is theimperfect coverage performance in a cellular network. The existing cellplan, with its location of access points (base stations), cannot providecell-wide coverage, at points within the desired coverage area, or atthe border of the desired coverage area, or both.

SUMMARY

An object with the present invention is to provide an apparatus, acommunication network, and a method and computer software that willimprove coverage performance in a cellular communication system comparedto prior art techniques.

A solution to this object is provided by a sub-node, such as a repeaterstation or relay station, equipped with a multi-beam sub-node linkantenna. The beams of the sub-node link antenna allow communicationbetween the sub-node and at least two nodes (access points/basestations). The sub-node is further provided with a control mechanismthat selects one of the nodes to extend the geographical primarycoverage area of the node to include at least one of the geographicalsecondary coverage areas of the sub-node.

An advantage with the present invention is that a network with aflexible coverage may be achieved.

Another advantage is that simultaneous connections to multiple nodesprovides the operator with the possibility to adaptively control whichnode(s) should serve the area covered by the sub-node.

Another advantage is that the sub-node may automatically adapt tovariations in the different communications links in a system with cellsize variations, such as cell breathing in CDMA systems, or otherwisevarying properties of the communications links between the sub-node andthe multiplicity of nodes connected to the sub-node via the linkantenna(s), thereby providing optimum performance in the area covered bythe sub-node.

Another advantage is that a remotely-controlled version of the sub-nodemay be used by the operator to offer the best possible cell plan, forthe available nodes and given the antenna coverage of the sub-node, atany given instant in time, which allows for load-balancing betweennodes.

Further objects and advantages are apparent for a skilled person fromthe detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show communication networks using prior art repeaters toimprove coverage performance.

FIG. 3 shows a first embodiment of a communication system according tothe invention.

FIG. 4 shows a second embodiment of a communication system according tothe invention.

FIG. 5 shows an embodiment of a sub-node according to the invention.

FIG. 6 shows a flow chart of the sub-node operation.

FIG. 7 shows a more detailed flow chart of the sub-node operation.

FIGS. 8 a-8 c illustrates how a coverage area of a sub-node according tothe invention changes dependent on traffic load.

FIG. 9 shows an alternative embodiment of a communication systemimplemented as a LAN for computer communication.

DETAILED DESCRIPTION

The invention is directed towards a repeater station or a relay stationfor use in a cellular communication network having one or more accesspoints (base stations). In the claims, node is used for an access pointand sub-node is used for a repeater station or a relay station. In thisapplication a repeater station is considered to only amplify thereceived signals (both from a terminal situated in a coverage area ofthe repeater station and from a node). A relay station is considered tocontain more functionality.

The principle idea of the invention is illustrated in FIG. 3 for thecase of a sub-node with a circular cylindrical sub-node multi-beam linkantenna. In addition to the advantages offered by conventional repeatersolutions (for example no need for wireline or wireless transmission ofbackbone-type data, such as IP traffic, to a site, and no need forcomplete base station equipment), the present invention provides forsignificantly better performance by exploiting the availability (throughthe multi-beam link antenna) of a multiplicity of signals from differentaccess points, i.e., nodes.

A major problem facing solutions based on single-antenna (single-beam)sub-nodes was introduced above, namely cell breathing. A multi-beamsub-node, on the other hand, is particularly well suited to handle cellbreathing, since it has simultaneous connections to multiple accesspoints via the multi-beam link antenna. The sub-node can enhance thepositive aspects of cell breathing, for example load-balancing betweencells in a CDMA system, by controlling which of the access points shouldbe associated with the “sub-node coverage antenna”, i.e., the antennawhose beam defines the coverage area of the sub-node.

FIG. 3 shows a first embodiment of a communication system 10, which issimplified and only comprises two nodes (access points) 11 _(A), 11_(B), each provided with an antenna system 3 _(A), 3 _(B) that generatescoverage to a geographical primary coverage area “A” and “B”,respectively. Omnidirectional antenna systems are in this embodimentused for the node antenna systems 3 _(A) and 3 _(B). A sub-node(repeater or relay station) 15 is in this embodiment provided with acircular cylindrical sub-node link antenna 16 and a coverage antenna 17that generates coverage in a secondary coverage area “a”. The secondarycoverage area “a” is indicated using a dashed line 18, and a userterminal 8 is situated within the secondary coverage area “a” andcommunicates with the sub-node 15, as indicated by the dashed arrows 9.The link antenna 16 is a multi-beam link antenna that communicates withboth node 11 _(A) and 11 _(B).

The secondary coverage area “a” remains in this example unchanged overtime. However, the primary coverage area “A” and “B” may change overtime and this is illustrated by arrows 14, and a dash-dot line 12 _(A),13 _(B) and a continuous line 13 _(A), 12 _(B) around the nodes 11 _(A)and 11 _(B), respectively.

To illustrate the operation of the invention, it is assumed that theprimary coverage area of each node is as indicated by the dash-dot lines12 _(A), 13 _(B). The primary coverage area “B” is larger than theprimary coverage area “A”, and the signals received by the sub-node linkantenna 16 from node 11 _(B) are stronger, or have better C/I(carrier-to-interference ratio), than the signals received from node 11_(A). The sub-node 15 includes a control mechanism, as described in moredetail below, that either automatically selects which node tocommunicate with, or is manually controlled by an operator. In thisexample, the sub-node 15 selects to communicate with node 11 _(B) asindicated by the dash-dot arrows 19 _(B). Thus, the user terminal 8 hasan established communication link with node 11 _(B), since the coverageof node 11 _(B) has been extended to include the secondary coverage area“a”.

At a later point of time, the primary coverage area “A” is larger thanthe primary coverage area “B” as indicated by the continuous lines 13_(A) and 13 _(B). The change in the size of the primary coverage areas“A” and “B” may be a deliberate change by the communication networkoperator to optimize the network capabilities, but may also be a changedue to cell breathing in a CDMA system. When the primary coverage area“B” is decreasing in size and the primary coverage area “A” isincreasing in size, the quality of the signals, as measured by forexample power level or pilot C/I, received by the sub-node link antenna16 from node 11 _(A) increases compared to the signals received fromnode 11 _(B). The sub-node 15 then selects, as illustrated in connectionwith FIGS. 6 and 7, to communicate with node 11 _(A), instead of node 11_(B), as indicated by the continuous arrows 19 _(A).

The process of selecting which access point's signals are “repeated” or“relayed” can be based on a number of control mechanisms. For example, apower meter may gauge the incoming (downlink) power from the accesspoints and the sub-node may then select the access point (node) with thehighest power. This process can involve a number of specific componentsand methods. In the case of an FDD (frequency-division duplex) system, anarrowband filter may be used to only extract downlink signals for theoperator using (owning) the sub-node. One filter will typically berequired for each of the beams pointing towards the available accesspoints, such that the power meter (one per beam) only measures the levelof the operator's signals. The power meter may report instantaneous or,maybe more desirable, time-averaged power level, which can be used by acontrol unit as one of (potentially) many quantities on which to basethe decision of which access point's signals should be “repeated”. Thehardware realization of the sub-node beam selection can be based onusing a switch between the multi-beam sub-node link antenna and therepeater coverage antenna. A gradual up- and down ramping of signalscorresponding to different access points is another possibility. In thecase of a CDMA system, the terminals (user equipments) in the areacovered by the repeater may then temporarily be in handover.

The sub-node may also be remotely controlled, thus allowing performanceoptimization from an operations center. In this case, the operator canadaptively change the cell plan to best suit the traffic situation. Forinstance, if an access point's signals are repeated and this accesspoint becomes heavily loaded (many users or high data rates), theoperator can command the sub-node to switch to another access point ofthe multiplicity of access points available to the sub-node.

Another problem that can be handled by the repeater is variations in thequality of the communications links between the sub-node and the accesspoints. These variations may be of a “short-term” nature, for exampledue to precipitation, or of a “longer term” nature, for example causedby changes in the propagation environment not envisioned at the time ofthe sub-node installation.

FIG. 4 shows a second embodiment of a communication system 20 having amore complex structure to illustrate further advantageous features ofthe present invention. Five nodes 11 _(A-E) acting as access points fortwo sub-nodes 15, 25 are illustrated. Each node has an antenna system 3_(A-E), and generates a primary coverage area “A”-“E” that may vary insize as indicated by arrows 24 over time. Dash-dot lines 22 _(A-E)indicate an initial primary coverage area for each node, and continuouslines 23 _(A-E) indicate changed primary coverage area for each node atan other point of time. A first sub-node 15 has a coverage antenna 17,which generates coverage to only one secondary coverage area “c”, and acircular cylindrical multi-beam sub-node link antenna 16, which receivessignals from at least two access points (nodes) 11 _(A), 11 _(B). Acommunication link is established between a terminal 8 _(c) situated inthe secondary coverage area “c” and a selected node, as described inconnection with FIG. 3.

A second sub-node 25 has a coverage antenna 27, which generates coverageto two secondary coverage areas “a” and “b”, and two multi-beam sub-nodelink antennas 26 _(A), 26 _(B), each receive signals from at least twoaccess points (nodes) 11 _(A), 11 _(B), and 11 _(C), 11 _(D), 11 _(E),respectively. A communication link between a terminal 8 _(b) situated inthe secondary coverage area “b” and a selected node is described inconnection with FIGS. 6 and 7. It is of course possible to usesingle-beam link antennas instead of multi-beam link antennas toestablish communication between the sub-node and one of the accesspoints, but it is preferred that multi-beam link antennas are used.

Dash-dot arrows 28 are used to indicate two-way communication for theinitial primary coverage areas, as illustrated by dash-dot lines 22_(A-E). A first dash-dot arrow 28 _(a) indicates two-way communicationbetween the second sub-node 25 and a selected node 11 _(A) to extend thecoverage area of the node to include the secondary coverage area “a”,and a second dash-dot arrow 28 _(b) indicates two-way communicationbetween the second sub-node 25 and a selected node 11 _(C) to extend thecoverage area of the node to include the secondary coverage area “b”. Athird dash-dot arrow 28 _(c) indicates two-way communication between thefirst sub-node 15 and a selected node 11 _(A) to extend the coveragearea of the node to include the secondary coverage area “c”.

In similar fashion, continuous arrows 29 are used to indicate two-waycommunication between sub-nodes and nodes when the primary coverageareas have changed as indicated by the continuous lines 23 _(A-E). Afirst continuous arrow 29 _(a) indicates two-way communication betweenthe second sub-node 25 and a selected node 11 _(D) to extend thecoverage area of the node to include the secondary coverage area “a”,and a second continuous arrow 29 _(b) indicates two-way communicationbetween the second sub-node 25 and a selected node 11 _(A) to extend thecoverage area of the node to include the secondary coverage area “b”. Athird dash-dot arrow 29 _(c) indicates two-way communication between thefirst sub-node 15 and a selected node 11 _(B) to extend the coveragearea of the node to include the secondary coverage area “c”.

FIG. 5 shows a sub-node 30 comprising two single-beam link antennas 31and 32 configured to receive signals transmitted from coverage antennasat node 1 and node 2. Each signal is forwarded via a first duplexer 33_(A) to a continuously variable low noise amplifier 34 which is providedfor each received signal S1 and S2. The amplified signals are fed, via asecond duplexer 33 _(B) into a power combiner 35, e.g. a continuouslyvariable power combiner (CVPC) or fixed power combiner (FPC). The powercombiner 35 is controlled by a control unit 36, and the receivedamplified signals are in this embodiment combined into a sub-nodetransmit signal which is transmitted to a terminal 8 present in asecondary coverage area using a sub-node coverage antenna 37. A loadimpedance 38 is also provided to take care of excess power in the powercombiner 35.

The control unit is provided with data to control the operation of thesub-node 30. The data could be provided directly from a network operatorin the case where the operation of the sub-node is manually controlled,or provided from nodes, operation maintenance centers (OMC), etc. in thecase where the operation of the sub-node is automatically controlled.There is also a possibility that the sub-node 30 gather information fromneighboring nodes and terminals present within the secondary coveragearea, and using the gathered information control the operation of thesub-node. However, this requires a more sophisticated and complexsub-node, such as a relay station.

The power combiner 35 will combine the received signals and weight thesignals in accordance to the control signal from the control unit 36.Optionally, the control unit 36 may control the gain in the low noiseamplifiers 34 to weight the received signals before the power combiner35 combines them into the signal that will be transmitted from thecoverage antenna 37. The power combiner 35 could also be provided withphase shifters and switches to select a secondary coverage are if thesub-node is designed to service more than one secondary coverage area.

A signal transmitted from the terminal 8 is received at the sub-nodecoverage antenna 37, through the power combiner 35 and divided into oneor two signals, depending on the control signals from the control unit36. Each signal is fed via the second duplexer 33 _(B) to a low noiseamplifier 39. Each amplified signal is fed to link antenna 31 and 32,respectively, via the first duplexer 33 _(A). The signal is thereaftertransmitted and received by the coverage antenna at Node 1 and Node 2.

The duplexer is normally used in system employing FDD (frequencydivision duplex), but in TDD (time division duplex), each duplexer isreplaced by a switch to perform the same function.

The described sub-node in FIG. 5 has only one secondary coverage areaand continuously selects which of the primary coverage area of the twonodes should be extended to include the secondary coverage area. Thismay be generalized to two or more secondary coverage areas. For highernumbers of secondary coverage areas and/or nodes, the block diagrambecomes more complex, but still realizable using the same types ofcomponents and optionally includes additional phase shifters andswitches.

The coverage antenna 37 may be an adaptive antenna that may change thenumber of beams and adapt the geographical coverage area provided byeach beam as shown in FIGS. 8 a-8 c and 9.

FIG. 6 shows a flow chart for operation of a sub-node, preferablyimplemented as a computer program installed in the control unit 36 andexecuted by a microprocessor in the sub-node 30 described above. Theprocess starts in 60 and a plurality of nodes with a primary coveragearea (CA) are provided in the first step 61 together with providing atleast one sub-node with a secondary coverage area. The nodes willfunction as access points for the sub-node, and the sub-node may be arepeater station or a relay station, as described above.

In the next step 62, the sub-node determines available nodes, i.e., thesub-node is provided with one or more link antennas to receive signalsfrom at least two nodes (access points), and a control unit determineswhich of the received signals have an acceptable signal strength andquality to be qualified as an available node.

Only one of these available nodes may be selected, which is performed instep 63. There are several possible implementations of a controlmechanism for selecting a node, and these are described in more detailin connection with FIG. 7. Irrespectively how this is performed, theprimary coverage area of the selected node is thereafter extended toinclude the secondary coverage area of the sub-node in step 64. The flowends in 65.

FIG. 7 will describe in more detail how the selection of a node may beperformed and the flow continues from step 62 in FIG. 6, and independency of whether the selection of a node is going to be made by anoperator (manually) or be made automatically by measuring parameters inthe communication network, the flow continues via step 71 to step 72(manual operation) or to step 73 (automatic operation).

In the manual mode, step 72, data is sent to the sub-node containing theselected node and the secondary coverage area to which the primarycoverage area of the selected node is going to be extended. If thesub-node only has one secondary coverage area, this information is notneeded to extend the primary coverage area of the selected node toinclude the secondary coverage area of the sub-node. The flow continuesthereafter in step 77.

In the automatic mode, steps 73-76, desired parameters, such as trafficload, signal quality, power level, etc., are measured in step 73.Example of measurable parameters that reflects signal quality includesbit rate, bit error rate, frame error rate, signal-to-noise-ratio (SNR),etc. The desired parameter is preferably measured on a pilot signal ortraffic channel signals. Dependent on the measured parameters in step73, a node is selected in step 74. If there is only one secondarycoverage area of the sub-node, the flow continues to step 77 via step75. In case of a plurality of secondary coverage areas of the sub-node,the flow continues to step 76 via step 75.

One or more of the secondary coverage areas of the sub-node is selectedin step 76, to which the primary coverage area of the selected nodeshall be extended. This may be implemented as a time dependent selectionprocess in which a predetermined time table is set-up to optimize theusage of the available capacity dependent on the time of day. This maybe implemented on a time slot level, where different time slots from anode is transmitted in different secondary coverage areas that areavailable at the sub-node. In an alternative solution, all signals froma node are transmitted in a first secondary coverage area during apredetermined time interval, e.g. between 6 am and 9 am, and then thesignals are transmitted in a second secondary coverage area during theremaining time period, i.e. 9 am to 6 am.

In a time independent selection process, the traffic load of theneighboring nodes that are in communication with the sub-node may bemeasured, and as a result from these measurements, the selection of asecondary coverage area is made, as described in more detail inconnection with FIGS. 8 a-8 c and 9.

In step 77, a check is performed to determine if there is a change inthe selected node that serves the secondary coverage area. If the samenode is selected to extend its coverage area to the secondary coveragearea, then the flow continues to step 79. In other cases, a hand-overneeds to be performed for all active user terminals present in thesecondary coverage area, step 78, before the flow continues to step 79.Steps 71-78 are repeated if the communication link should be updated,step 79, or else, the flow continues to step 65 in FIG. 6.

FIGS. 8 a-8 c illustrates how a coverage area of a sub-node 80 accordingto the invention changes dependent on a measured parameter, e.g. trafficload in the neighbouring nodes 81 and 82. FIG. 8 a illustrates how aprimary coverage area “B” of a first node 81 is extended to include asingle secondary coverage area “a” of the sub-node 80. Several activeuser terminals 8 (identified with crosses) are present within thesecondary coverage area “a” and the traffic load of either the firstnode 81 or the sub-node 80 is continuously monitored. When apredetermined traffic load level is reached, the secondary coverage area“a” is divided into two parts, coverage areas “a₁” and “a₂”, asillustrated in FIG. 8 b, and the active user terminals in one of thecoverage areas are transferred to a second node 82. The primary coveragearea “A” of the second node 82 is thus extended to include the dividedsecondary coverage area “a₁”, and the primary coverage area “B” isextended to only include the divided secondary coverage area “a₂”.

Normally this process would be enough to reduce the traffic load of thefirst node 81 if the active user terminals are spread out evenly withinthe secondary coverage area “a”. Unfortunately, in this example, themajority of the active user terminals 8 are situated within the dividedsecondary coverage area “a₂”, and thus the traffic load of the firstnode 81 still remains. Further actions are needed to redistribute thetraffic load to available nodes, i.e. to the second node 82. This may beperformed by further dividing the coverage area “a₂” into two parts,coverage areas “a_(2a)” and “a_(2b)”. The coverage area “B” of the firstnode 81 is then extended to include only one of these coverage areas,e.g. coverage area “a_(2b)”, and the coverage area “A” is extended toinclude the other part, e.g. coverage area “a_(2a)”, in addition tocoverage area “a₁”, see FIG. 8 c. A more even distribution of thetraffic load between the nodes 81 and 82 for active user terminalspresent within the secondary coverage areas is achieved.

In an alternative example, the size of the secondary coverage areasillustrated in FIG. 8 b could be altered by shaping the beams thatgenerate the secondary coverage areas “a₁” and “a₂”. It is possible toalter the beam shape in such a way that the modified coverage area “a₁”will cover the area of the combined coverage areas “a₁” and “at_(a)”,and the modified coverage area “a₂” will only cover area “a_(2b)”.

The operation for dividing the secondary coverage area into smallerparts and altering the beam shape to generate different size of thecoverage area is well known to a skilled person in the art from thedescribed antenna system in U.S. Pat. No. 6,246,674 by Feuerstein etal., which is incorporated by reference.

FIG. 9 shows an alternative embodiment of a communication system 90implemented as a network for communication, such as LAN, WLAN, GSM, 3G,etc. In the embodiment three nodes 91, 92 and 93 (base stations), whichare connected to form said communication network, are provided togetherwith three sub-nodes 94, 95 and 96 (relay or repeater stations) within abuilding 99. The continuous lines represent the primary coverage areas“A”, “B” and “C” of the nodes 91, 92 and 93, respectively. The dashedlines represent the extent of the secondary coverage areas “a”, “b” and“c” of the sub-nodes 94, 95 and 96, respectively.

Continuous arrows 97 _(a), 97 _(b) and 97 _(c) illustrates how userterminals present in the respective secondary coverage areas “a”, “b”and “c” are in communication with node 93 at a first point of time. Node91 and node 92 only service user terminals within their primary coveragearea “A” and “B”, respectively.

User terminals may move within the building and maintain communicationwith the network, which will shift the traffic load between the nodes.As an example, at a second point of time, user terminals may have movedfrom the primary coverage area “A” and “B” into the hallway covered bysub-node 94. The traffic load at node 93 will increase since the userterminals within the secondary coverage area “a” are in communicationwith node 93 (continuous arrow 97 _(a)). To be able to maintain goodoperation for the user terminals the secondary coverage area “a” isdivided into two secondary coverage areas “a1” and “a2”, as illustratedby the dash-dot lines and described in connection with FIGS. 8 a-8 c.

The primary coverage area “A” is extended to include secondary coveragearea “a1” and primary coverage area “B” is extended to include secondarycoverage area “a2”. For systems supporting it, handover between node 93and nodes 91 and 92 may naturally be performed in order to not disruptthe communication link for each active user terminal within thesecondary coverage areas of sub-node 94. Dash-dot arrows 98 _(a1), 98_(a2), 98 _(b) and 98 _(c) illustrates how user terminals present in therespective secondary coverage areas “a1”, “a2”, “b” and “c” are incommunication with nodes 91, 92 and 93 at the second point of time.

A further decision basis for coverage area association, the effectivedistance to active users within a secondary coverage area may bedetermined by measuring time-delay at the node, which primary coveragearea is extended to include the secondary coverage area.

Single or double polarized antennas, as well as any type of diversityantennas, may naturally be used to implement the present invention, andscope of the claims should cover any type of antenna systems that areused for communication purposes between nodes and sub-nodes.

The invention is applicable for mobile communications systems, such asWCDMA, CDMA2000 and WiMAX802.16e, as well as for fixed communicationssystems, such as WiMAX802.16d.

1. A sub-node configured to be used in a cellular communication network,said sub-node comprising: at least one transmitting and receivingcoverage antenna to generate coverage in at least one geographicalsecondary coverage area, said sub-node configured to be in communicationwith a node, generating coverage in a geographical primary coveragearea, to extend the coverage of said node to include said at least onegeographical secondary coverage area; at least one link antennaconfigured to provide communication with a plurality of nodes: means todivide the at least one geographical secondary coverage area of said atleast one sub-node into at least two parts dependent on measuredparameters, wherein said parameters include at least the traffic load inthe cellular communication network; and, a control mechanism, responsiveto said measured parameters, configured to select a first node and asecond node from said plurality of nodes to extend coverage of each ofsaid selected first node and second node to include at least one part ofsaid divided geographical secondary coverage area.
 2. The sub-nodeaccording to claim 1, wherein said means to divide the at least onegeographical secondary coverage area comprises means to shape the beamsgenerating the divided secondary coverage areas.
 3. The sub-nodeaccording to claim 1, wherein said sub-node is a relay station orrepeater station.
 4. The sub-node according to claim 1, wherein saidcontrol mechanism is manually controlled by an operator managing thecellular communication network.
 5. A cellular communication networkcomprising a plurality of nodes each having transmitting and receivingantennas to generate coverage in a geographical primary coverage area,said network comprising: at least one sub-node having transmitting andreceiving coverage antennas to generate coverage in at least onegeographical secondary coverage area, said sub-node in communicationwith a node to extend the coverage of said node to include said at leastone geographical secondary coverage area; and, at least one link antennaproviding communication with a plurality of nodes; wherein said at leastone sub-node further comprises: means to divide the at least onegeographical secondary coverage area of said at least one sub-node intoat least two parts dependent on measured parameters, wherein saidparameters include at least the traffic load in the cellularcommunication network; and, a control mechanism, responsive to saidmeasured parameters, configured to select a first node and a second nodefrom said plurality of nodes to extend each coverage of said selectedfirst node and second node to include at least one part of said dividedgeographical secondary coverage area.
 6. The cellular communicationnetwork according to claim 5, wherein said means to divide the at leastone geographical secondary coverage area comprises means to shape thebeams generating the divided secondary coverage areas.
 7. The cellularcommunication network according to claim 5, wherein at least one of saidat least one sub-node is a relay station or a repeater station.
 8. Thecellular communication network according to claim 5, wherein saidcontrol mechanism is manually controlled by an operator managing thecellular communication network.
 9. The cellular communication networkaccording to claim 5, wherein said node is a base station.
 10. A methodfor generating coverage in a cellular communication network comprising aplurality of nodes each having transmitting and receiving antennas togenerate coverage in a geographical primary coverage area, said methodcomprising the steps of: providing at least one sub-node havingtransmitting and receiving coverage antennas generating coverage in atleast one geographical secondary coverage area, said sub-nodecommunicating with a node to extend the coverage of said node to includesaid at least one geographical secondary coverage area; providingcommunication between said at least one sub-node and a plurality ofnodes; dividing the at least one geographical secondary coverage area ofsaid at least one sub-node into at least two parts dependent on measuredparameters, wherein said parameters include at least the traffic load inthe cellular communication network; and, configuring a controlmechanism, responsive to said measured parameters, to select a firstnode and a second node from said plurality of nodes to extend eachcoverage of said selected first node and a second node to include atleast one part of said divided geographical secondary coverage area. 11.The method according to claim 10, further comprising the step ofdividing said at least one geographical secondary coverage area byshaping the beams generating the divided secondary coverage area. 12.The method according to claim 10, further comprising the step ofselecting at least one of said at least one sub-node to be a relay orrepeater station.
 13. The method according to claim 10, wherein saidcontrol mechanism is manually controlled by an operator managing thecellular communication network.
 14. The method according to claim 10,wherein said node is a base station.